In recent years, slave devices that have penetrated the market use large-capacity nonvolatile storage elements, such as flash memories, and process data at a high speed. Examples of such slave devices include secure digital (SD) cards and memory sticks. The slave devices are used in, for example, personal computers, personal digital assistants (PDAs), mobile telephones, digital cameras, audio players, and car navigation systems, which function as host devices that can use the slave devices.
In addition to the memory function, the slave devices that have been commercialized recently can also have, for example, wireless LAN functions, network connection functions for wireless communication based on Bluetooth (trademark), positioning functions using the global positioning system (GPS), and input and output functions such as one-segment broadcasting reception functions for terrestrial digital TV broadcasting. Host devices are connected to these slave device to use the functions of the slave devices.
A communication system that includes such a host device and such a slave device tends to process an increasingly larger amount of data every year. This has generated a market demand for increasing the processing speed of an interface between the host device and the slave device. There also is a market demand for maintaining compatibility with the existing interfaces that are widely used in the market so that the existing interfaces will be usable continuously.
To increase the processing speed of the interface while allowing the existing interface circuit (interface device) to be usable continuously, an interface voltage (voltage used in the interface device) needs to be lowered.
With a conventional technique for selecting an operating voltage of a host device and a slave device from a plurality of interface voltages, the host device and the slave device may have a determination key indicating the level of an interface voltage, and the host device may detect an interface voltage of the slave device and then switch the voltage to be supplied to the slave voltage to determine the interface voltage (see, for example, Patent Citation 1).    Patent Citation 1: Japanese Unexamined Patent Publication No. 2002-169631